Owing to its outstanding physical and chemical properties, including excellent heat resistance, mechanical strength and resistance to radiation, silicon carbide (SiC) has attracted attention as an environmentally rugged semiconductor material. Moreover, recent years have seen increasing demand for SiC single-crystal substrates for use as substrate wafers in short wavelength optical devices in the blue-to-UV spectral region, high-frequency electronic devices, high-breakdown voltage electronic devices and the like. However, no crystal growth technology enabling reliable industrial-scale supply of large-area single-crystal SiC of high-quality has yet been established. Practical utilization of SiC has therefore been thwarted despite these many merits and its high potential.
Single-crystal SiC of a size usable for fabrication of semiconductor devices has up to now been conducted on a laboratory scale using, for example, the sublimation recrystallization process (Lely process). However, the single crystal obtained by this method is of small area, and its dimensions and shape are difficult to control. Moreover, control of the crystal polytype and doping carrier concentration of the SiC is not easy either. On the other hand, cubic single-crystal SiC is being produced by heteroepitaxial growth, i.e., growth on a substrate of a different type like silicon (Si), using chemical vapor deposition (CVD). Although large-area single crystal can be obtained by this process, high-quality single-crystal SiC is hard to produce because the approximately 20% lattice-mismatch between the SiC and the substrate causes ready occurrence of stacking faults and crystal defects.
The modified Lely process, which conducts sublimation recrystallization using a single-crystal SiC substrate as a seed, was developed to overcome these problems (Yu. M. Tairov and V. F. Tsvetkov, Journal of Crystal Growth, vol. 52 (1981) pp. 146-150). The modified Lely process is in use at many research institutions. Owing to its use of a seed crystal, the process can control the crystal nucleation process and, by controlling the ambient inert gas pressure to around 100 Pa to 15 kPa, can control crystal growth rate and the like with good reproducibility.
The principle of the modified Lely process will be explained with reference to FIG. 1. The single-crystal SiC used as the seed crystal and the single-crystal SiC powder used as the feedstock are placed in a crucible (usually made of graphite) and heated to 2000 to 2400° C. in an argon or other inert gas atmosphere (133 to 13.3 kPa). A temperature gradient is established during the heating so as to make the temperature of the seed crystal slightly lower than the temperature of the feedstock powder. Owing to its concentration gradient (produced by the temperature gradient), the sublimated feedstock is dispersed toward and transported to the seed crystal. Single crystal growth occurs when the feedstock gas reaching the seed crystal recrystallizes on the seed crystal. The resistivity of the crystal can be controlled during the growth by adding a doping gas to the inert gas atmosphere or by mixing a doping element or a compound thereof into the SiC feedstock powder. Typical substitutional impurities used to dope single-crystal SiC include nitrogen (n type), boron (p type) and aluminum (p type). The modified Lely process makes it possible to grow single-crystal SiC while controlling its polytype (6H, 4H, 15R and other polytypes), shape, and carrier type and concentration.
Currently, 2-inch (50.8 mm) to 3-inch (76.2 mm) single-crystal SiC substrates are being produced by the modified Lely process and used for epitaxial film growth and device fabrication. However, fabrication of high-performance devices has been hindered by the presence in the single-crystal SiC substrates of dislocations (line-like crystal defects) at the rate of several tens of thousands to several millions per square cm. Reduction of basal plane dislocations present on the (0001) basal plane is particularly desirable because they are known to degrade the reliability of SiC devices.
It has been reported that basal plane dislocations occur and proliferate as the result of thermal stress sustained by the crystal during growth. During crystal growth, thermal stress acting on the basal plane dislocations introduced into the single-crystal SiC causes them to slip, and the slipping promotes occurrence of Frank-Read type dislocations. The proliferation of these basal plane dislocations greatly increases the basal plane dislocation density in the single-crystal SiC. The etch pit density attributable to basal plane dislocations measured on the single-crystal SiC substrate at an off-angle of 8° from the (0001) Si plane is ordinarily well over 1×104 cm−2.